Light disinfection system and method

ABSTRACT

A lighting system includes a light source configured to generate light toward one or more surfaces or materials to inactivate one or more pathogens on the one or more surfaces or materials. The light includes an inactivating portion having wavelengths in a range of 280 to 380 nanometers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending, prior-filed, commonlyowned U.S. application Ser. No. 15/065894, filed 10 Mar. 2016, which ishereby incorporated by reference. The latter application claims priorityto U.S. Provisional Application No. 62/134,954, which was filed on 18Mar. 2015, and the entire disclosure of which is incorporated herein byreference.

FIELD

Embodiments of the subject matter disclosed herein relate to lightingsystems that disinfect surfaces and materials of pathogens (i.e.,inactivate the pathogens).

BACKGROUND

Light has been used to disinfect pathogens on surfaces, in air, or inwater (i.e. to inactivate pathogens in fluids, or on solid surfaces).Herein the term “pathogen” refers to any microscopic organism capable ofcausing disease or infection in a human. These include bacteria,viruses, spores, and fungi. Herein the term “inactivate” refers torendering a pathogen inactive, or unable to infect a human. This mayinclude killing the pathogen, rendering it unable or less able toreplicate, or rendering it unable to infect a human. Some known systemsuse ultraviolet (UV) light to inactivate pathogens. Herein, we adopt thestandard definitions, as follows: ultraviolet light refers to lighthaving wavelengths in the range 100 nanometers (nm) to 400 nm; the foursub-ranges within the UV range include the Vacuum UV from 100 to 200 nm;UVC from 200 to 280 nm; UVB from 280 to 315 nm; and UVA from 315 to 400nm. Although the visible colors are not rigorously defined in thelighting industry, herein we adopt commonly used definitions for violetand blue as comprising about 400 to about 450 nm, and about 450 to about490 nm, respectively. Some known systems use UV light in the range of200 to 300, including the UVC range and some of the UVB range, toinactivate pathogens by damaging their DNA or RNA and rendering themincapable of reproduction, thus incapable of causing disease in humans.This range of 200 to 300 nm is referred to in the literature, andherein, as the germicidal range. Light sources such as low-pressure andmedium-pressure mercury lamps, and pulsed xenon lamps are known toinactivate pathogens by irradiation of fluids (e.g. water or air orother fluids) and surfaces with wavelengths in the germicidal range.

A description, with references, of disinfection lighting using thegermicidal range of light is found in a recent publication “Light basedanti-infectives: ultraviolet C irradiation, photodynamic therapy, bluelight, and beyond”, Yin et al., Curr Opin Pharmacol, 2013 October. Lightof these wavelengths can have high inactivation rates for many types ofpathogens on surfaces, in air, or in water. But, exposure to light ofthese germicidal wavelengths can be hazardous to human beings. As aresult, these systems may only be used safely in locations where humanbeings are not present, or are prevented from accessing.

Other systems may use violet, blue, or longer wavelengths of visiblelight to inactivate most common pathogens, but the inactivation rates ofthe visible wavelengths have been found to be three to five orders ofmagnitude lower than for the germicidal range of wavelengths of light.

U.S. Pat. No. 8,398,264 describes a lighting device that emits visiblelight at a wavelength and irradiance sufficient to inactivate one ormore pathogenic bacterial species. U.S. Pat. No. 9,039,966 describes amethod wherein the visible light for inactivating the MRSA pathogenincludes wavelengths in the range of 400-420 nm, i.e., violet light. Butthese visible light, especially violet light, systems have severalproblems: disinfecting pathogens with visible light requires a verylarge flux density of light (e.g. about 0.5 to about 5 W/m²) incidentfor several hours on the surface to be disinfected; if violet light isused for disinfection, the amount of electrical power required tooperate the visible LEDs at sufficient dose to inactivate about 90-99%of a population of common pathogens is so high that the overall efficacyof the lighting system is significantly reduced, by as much as about 10%to about 50% or more; if violet or blue light is used for disinfection,the flux of violet light in the space occupied by humans is so largethat some occupants suffer eyestrain, headaches, nausea, dizziness ordiscomfort; if violet or blue light is used for disinfection, the fluxof violet or blue light is so large that it greatly distorts the colorpoint of the white light with which it might be mixed, and is so largethat the flux may not be substantially increased for the benefit of moreeffective disinfection without exceeding the permissible limit of theblue light photobiological hazard standard, rendering the light sourceunsafe for humans. The limited magnitude of disinfection is aproblematic limitation of violet light. The 90-99% inactivation istypically achieved only under certain favorable conditions fordisinfection of an architectural space, including the following factors:vegetative bacteria, possibly excluding spores and viruses; in directline-of-sight of, and in sufficient proximity to the disinfecting lightsource; and absence of biofilm; with significantly lower inactivationrates for spores and viruses. In most non-ideal circumstances, theinactivation rate may be considerably less than about 90-99%, and maytherefore be ineffective, e.g. under circumstances of lower flux levelsdue to shadowing or distance from the disinfecting light source; biofilmor high bio-burden; spores, or viruses. As a result, these systems areexpensive, energy-inefficient, visually obtrusive, physiologicallydisturbing to some individuals, marginally safe for human exposure, andlimited in the magnitude of disinfection by the compromises in systemdesign required to overcome these problems. The term “common pathogen”herein refers to a pathogen that is commonly responsible for humandisease, especially in the context of the most commonly encounterednosocomial infections, so-called hospital acquired infections (HAI),including the well-known pathogens Staphylococcus aureus (S. aureus);Methicillin-resistant Staphylococcus aureus (MRSA); Clostridiumdifficile (C. diff.).; Escherichia coli (E. coli.); and several othergram-positive, gram-negative, spore, viral, and fungal pathogens.

Other systems may use visible light having wavelengths centered on 405nm light to provide inactivation of about 90-99% of a pathogenpopulation for many common pathogens, but only if the light sourcegenerates the disinfecting light for extended periods of time (i.e.,five to ten hours or more of exposure time), and if the disinfectinglight is generated at significantly large radiant power densities. Thedose of light for inactivation of about 90% of a population of commonpathogens using 405 nm light is typically about 10-20 J/cm², per thereference Maclean et al., High-Intensity Narrow-Spectrum LightInactivation And Wavelength Sensitivity Of Staphylococcus Aureus, FEMSMicrobiol Lett 285 (2008) 227-232. This corresponds to an irradiance of3.5-7 W/m² of 405 nm light, for an exposure time of 8 hours. Given thetypical efficiency of 405 nm LEDs today of about 20-30% (efficiency ofconverting electrical power to radiated optical power) the disinfectionlighting requires an electrical power density of about 12-35 W_(el)/m².The electrical power density used for general white-light illuminationat a level of 500 lux from a light source having a typical efficacy of100 LPW is about 5 W_(el)/m². If the disinfection lighting, providingabout 90-99% disinfection on a target surface is added to, and mixedwith, the white lighting having a flux density of about 500 lux on thetarget surface, then the electrical power density of the combinedlighting system will be about 17-40 W_(el)/m² Since the electrical powerdensity required for disinfection using 405 nm light is much greaterthan that required for white-light illumination, the overall systemefficacy of the illuminating and disinfecting lighting system may bereduced by as much as about 70-90%, from typically 100 LPW to about10-30 LPW.

The American Society of Heating, Refrigerating, and Air-ConditioningEngineers (ASHRAE) Energy Standard 90.1-2013 provides an upper limit forthe Lighting Power Density (LPD) in the range of about 0.5 to about 2.0W_(el)/ft², or about 5 to about 20 W_(el)/m² in typical indoor lightingapplications. Specifically, for a hospital, the upper limit LPD allowedis 1.05 W_(el)/ft², or 11.3 W_(el)/m², calculated by the Building AreaMethod. For the typical values given above for a disinfection lightingsystem using 405 nm light, the LPD of about 17-40 W_(el)/m² exceeds theASHRAE limit of 11.3 W_(el)/m². The ASHRAE limit would also constrainthe flux of the disinfection portion of the lighting (the 405 nmradiation) to no more than about 6 W_(el)/m² which is insufficient toinactivate 90-99% of pathogens over a period of 8 hours using 405 nmlight. The ASHRAE limits may adversely affect the ability of customersto use 405 nm disinfection lighting in some regulated applications.

United States Patent Application No. US2011/0251657 Al describes alighting device that emits visible light and UVA light in the range320-380 nm at an irradiance that is in the range of 3 to 15% of theirradiance of the visible light, wherein the visible light provides 700lux at the work surface, sufficient to activate the human serotoninnervous system with the advantage of decreasing aggressiveness inhumans. When the radiant energy of the near ultraviolet radiation with awavelength of 320 nm or longer, but shorter than 380 nm, is less than 3%of the radiant energy of the visible light, advantageous effects on theserotonin nervous system would not be obtained. Since the visible lightused provided 700 lux, and since 3% or more of the radiant energy in theUVA is required to activate the serotonin nervous system, then it may beexpected that about 4% or more of the radiant energy should be emittedin the UVA if the visible light component is only 500 lux, instead of700 lux in order to activate the human serotonin nervous system.

BRIEF DESCRIPTION

In one embodiment, a lighting system includes a light source configuredto generate light toward one or more surfaces or materials to inactivateone or more pathogens on the one or more surfaces or materials. Thelight includes an inactivating portion having wavelengths in a range of280 to 380 nanometers.

In one embodiment, a method for inactivating one or more pathogens andoptionally concurrently illuminating a room having one or more humanoccupants while the pathogens are inactivated is provided. The methodincludes generating light from a light source toward one or moresurfaces or materials to inactivate the one or more pathogens on the oneor more surfaces or materials. The light is generated with aninactivating portion of the light including wavelengths in a range ofabout 280 to about 380 nanometers.

In one embodiment, a lighting system includes a light source configuredto generate light toward one or more surfaces or materials to inactivateone or more pathogens on the one or more surfaces or materials. Thelight source is configured to generate an inactivating portion of thelight including wavelengths in a range of 280 to 380 nanometers,including no more than 0.001 watts of actinic ultraviolet light persquare meter of floor area, including no more than 10 watts per squaremeter of floor area of ultraviolet A light, and including no more than100 watts of blue light per steradian per square meter of floor area.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter described herein will be better understood fromreading the following description of non-limiting embodiments, withreference to the attached drawings, wherein below:

FIG. 1 illustrates a lighting system according to one embodiment of theinventive subject matter;

FIG. 2a illustrates a dataset of a dose of disinfecting light requiredfor about 99% inactivation of E. coli pathogens according to oneexample;

FIG. 2b illustrates a dataset of a dose of disinfecting light requiredfor about 99% inactivation of S. aureus pathogens according to oneembodiment of the inventive subject matter;

FIG. 2c illustrates the dataset with a linear trend line from FIG. 2bwith several lines representing the dose vs. wavelength obtained from afirst-order kinetic model according to one example;

FIG. 2d illustrates the dataset with the linear trend line fit from FIG.2b with the lines from FIG. 2c according to one example;

FIG. 3 illustrates several hazard functions;

FIG. 4 illustrates a bar graph of log-reduction at a wavelength of lightof 369 nm compared to a wavelength of 404 nm for several testedconditions according to one example;

FIG. 5 depicts photo-inactivation kinetics of light having wavelengthsof 369 nm, 388 nm, and 404 nm light after normalization to untreatedcontrols according to one example;

FIG. 6 illustrates a dataset of the reduction of S. aureus for a givendose of 23.5 J/cm² as a function of wavelength according to one example;

FIG. 7 illustrates total hemispherically 2n-integrated irradiance at areasonable average for the 48 contiguous states of the United States ofAmerica over a period of one year according to one example;

FIG. 8 illustrates the three color-matching functions according to the1931 CIE (the International Commission on Illumination) XYZ color space;

FIG. 9a illustrates the safety factors of exposure of humans to lightneeded for disinfection at various wavelengths for three hazardfunctions;

FIG. 9b illustrates the minimum safety factor of exposure of humans tolight needed for disinfection at various wavelengths; and

FIG. 9c illustrates the minimum safety factor of exposure of humans tolight disinfection at various wavelengths for a range of trend linefits.

DETAILED DESCRIPTION

One or more embodiments of the inventive subject matter described hereinemploy one or more light sources generating light that inactivatespathogens with an inactivation rate that follows a kinetic dependence,or relationship, between the energy of photons of the light and theinactivation rate. In one embodiment, this kinetic energy dependence, orrelationship, can provide for an inactivation rate (e.g., a rate atwhich pathogens are inactivated so that the pathogens are no longer ableor operable to cause disease in living organisms) that increases by afactor of ten for each 0.27 to 0.44 eV increase in the photon energy ofthe light. Therefore, an inactivation rate that is comparable to theinactivation rate achieved using light of about 405 nm (3.06 eV) can beachieved at a wavelength of about 355 nm (3.50 eV) to about 372 nm (3.33eV), corresponding to 0.44 to 0.27 eV increase per factor of 10inactivation rate increase, respectively, using only about 10% of thepower used for inactivating pathogens with 405 nm light. This can allowfor the electrical lighting power density (LPD) used in the light sourcefor inactivation of pathogens to be reduced (e.g., from about 12-35W_(el)/m² to about 4 W_(el)/m² or to about 1 W_(el)/m², or anothervalue) while increasing the efficacy of the combined illuminating anddisinfecting system relative to some known visible light disinfectionsystems from approximately 10-30 LPW to 80 LPW or higher.

To provide perspective on the scale of the irradiance in the prior artand in the present invention required to inactivate about 90-99% ofcommon pathogens, it is useful to compare those irradiances with theirradiance of the sun. At the top of the earth's atmosphere theirradiance (integrated over all wavelengths) from the sun is about 1400W/m², reduced to about 1100 W/m² at the earth's surface at sea level,with the sun at zenith. A standard solar irradiance spectrum has beendefined by the American Society for Testing and Materials document ASTMG173-03, which is incorporated herein by reference. The TerrestrialGlobal 37 degree Direct Normal+Circumsolar irradiance (i.e., totalhemispherically 2π-integrated irradiance at a reasonable average for the48 contiguous states of the United States of America over a period ofone year) vs. wavelength 710 is shown in FIG. 7, and is herein referredto as “solar irradiance”. Solar irradiance averages about 1.2 W/m²-nm inthe visible range, and about 0.35 W/m²-nm in the UVA range.Specifically, the irradiance is about 0.88 W/m²-nm at 405 nm, and about0.42 W/m²-nm at 365 nm. For an LED light source having a Gaussiandistribution of wavelengths characterized by a peak wavelength, and aspread of wavelengths defined by the full-width at half-maximum (FWHM),with FWHM equal to about 20 nm, the solar irradiance integrated over thewavelength distribution of a 405 nm LED is about 18 W/m², and for a 365nm LED is about 9 W/m². Therefore, the irradiance of about 4 W/m²required in the prior art to inactivate pathogens at about 405 nm isabout 4× weaker than the 18 W/m² solar irradiance at the earth's surfaceintegrated over the range of wavelengths emitted by the 405 nm LED; andthe irradiance of less than 1 W/m² required in the present invention toinactivate pathogens at about 365 nm is about 9× weaker than the 9 W/m²solar irradiance at the earth's surface integrated over the range ofwavelengths emitted by the LED. In this estimate, the prior art (andpresent invention) can be interpreted as being about 4× (and at least9×) safer than exposure to those same wavelengths from the sun. Sincecontinuous exposure to the sun is not considered to be safe for humans,this estimate is not sufficient to establish safety for human exposure.Rather, the actual photobiological hazard calculations will be providedin a later section. This estimate, however, does provide perspective tothe relatively low irradiation that is required for 90-99% inactivationof common pathogens using violet or UVA light. Both the prior art andthe present invention have been found to provide 90-99%, or more,inactivation of common pathogens using about 1 order of magnitude lessirradiance using LEDs than is provided by sunlight, when integrated overthe corresponding wavelength ranges.

Additionally, reducing the wavelength of light used to inactivatepathogens below about 380 nm, or a lower value, causes the inactivatingportion of the light to be less perceptible to human beings or a humanobserver of the light. FIG. 8 illustrates the three color-matchingfunctions—x function 810, y function 812, and z function 814—accordingto the 1931 CIE (the International Commission on Illumination) XYZ colorspace. They provide the numerical description of the chromatic responseof a standard observer, from which color responses of the human eye arecalculated. The strongest of the 3 functions in the violet and UVA rangeis the z function, which has a magnitude 820 of about 0.11 at 405 nm,but only a magnitude 830 of about 0.0011 at 365 nm, or 100 times smallerthan at 405 nm. Even though some humans are known to be able to perceivelight having wavelengths as short as about 310 nm, the average humanperception of light having wavelengths shorter than about 380 nm is verylow, and is exponentially diminishing vs. wavelength, so that the CIEstandards for calculating the human perception of light is quantified tobe zero at wavelengths shorter than 360 nm. Light at longer wavelengthsin the violet and blue (such as 405 nm) is visible to human beings, andit is known that high flux densities of violet or blue light can causeundesirable physiological effects, including nausea, dizziness, anddiscomfort. Using a shorter wavelength of light as described herein maysubstantially reduce the distortion of the color of the illuminant andthe lighted space, and the undesirable physiological effects.Optionally, the inactivating light source may be separate from thewhite-light source that illuminates the space, and since the relativeinvisibility of the inactivating light is less apparent, and lessdisturbing to the inhabitants, it may even be left on continuously withor without the white-light source.

An additional benefit of the shorter wavelengths of light used in one ormore embodiments of the inventive subject matter described herein isthat sufficient irradiance can be provided at these wavelengths toinactivate common pathogens, without exceeding safety limits for humanexposure to skin or eyes. There are currently six types ofphotobiological hazards for optical radiation in the range 200 to 3000nm (UV through infrared, or IR) that are covered by internationalstandards: Actinic UV, Near UV, Blue Light, Retinal Thermal, Cornea/LensIR, and Low Luminance Retinal IR. The first three hazards pertain tolight sources emitting in the blue, violet, and UV ranges, and must beaddressed in disinfection lighting systems operating in the blue,violet, or UV. A known visible light disinfection system uses a radiantpower density of about 0.5 to about 5 W/m², typically about 1 W/m², ofdisinfecting light having a wavelength centered on about 405 nm toachieve about 90-99% inactivation of a population of common pathogensafter about 5 to 10 hours of exposure is known to be safe relative toeach of the three blue, violet, and UV hazards. Of the threephotobiological hazards pertinent in the blue, violet, and UV ranges, asshown in FIG. 3, the UVA hazard function 320 is flat in the range 315 to400 nm, but only the actinic hazard function 310 increases at decreasingwavelengths below 405 nm. Therefore, it has been anticipated in theprior art that a disinfection light source operating at wavelengths inthe UV (i.e., below 400 nm) may be unsafe relative to the UVA or actinichazards. While it may be true that UVC or short-wavelength UVBirradiance sufficient to inactivate pathogens is unsafe for humans, wehave discovered that UVA and long-wavelength UVB radiation in a range ofabout 300 nm to about 400 nm that has sufficient irradiance toinactivate pathogens is safe for humans. We have discovered that thedecrease in wavelength required to increase the inactivation rate of S.aureus by 10× (i.e., 1-log) at a constant dose having magnitude ˜10J/cm² is about 32 nm in the range from about 405 nm to about 365 nm.This result implies that the irradiance at 365 nm required to provideabout 90-99% kill of S. aureus is about 18× lower than that required at405 nm. Of the three relevant action spectra for photobiological hazardsthe hazard that increases fastest vs. decreasing wavelength is theactinic hazard, which increases by only about 3.6× from 400 nm to 365nm. So, the radiant power density required for about 90-99% inactivationof common pathogens at 365 nm is actually about 5× (18× vs. 3.6×) saferon actinic hazard for human exposure than the radiant power densityrequired for about 90-99% inactivation of common pathogens at 400 nm.This is exactly the opposite of the trend that is taught in prior artwhich states that UV light is more hazardous than visible light, andtherefore using visible light for disinfection is safer for humans thanusing UV light. One of our discoveries is that the slope of theinactivation rate vs. wavelength greatly exceeds the slope of any of thephotobiological hazard functions as the wavelength of the disinfectinglight is reduced from the violet down through the UVA, and possibly evenshorter wavelengths. This discovery enables the inactivation of commonpathogens using UV light in the range of about 300 nm to about 400 nmwith the following advantages, relative to using light in the visiblerange, that have not been anticipated in the prior art: higherelectrical system efficiency; lower system cost; less distortion of thecolor point when mixed with white light; reduced or eliminatedphysiological disturbance to humans; greater photobiological safety forhumans; higher inactivation rate of pathogens.

FIG. 1 illustrates a lighting system 100 according to one embodiment ofthe inventive subject matter. The system 100 includes one or more lightsources 102 disposed in an environment 104 to be disinfected ofpathogens. The light sources 102 may be powered from a power source (notshown in FIG. 1), such as a utility grid, batteries, etc. In theillustrated embodiment, the environment 104 is a medical surgical suite,but optionally may be another environment, such as a hospital room, adoctor's office, a dentist office, a school or room in a school,bathroom, or a public area. The light source(s) 102 may include LEDsthat generate light toward one or more surfaces or materials 106 in theenvironment 104 to inactivate one or more pathogens on the one or moresurfaces or materials 106. Alternatively, the light source(s) 102 mayinclude one or more other devices that generate light. In oneembodiment, the surfaces or materials 106 may be solid objects, and maynot include water or air in one embodiment. For example, the lightgenerated by the light source(s) 102 may inactivate pathogens on floors,walls, and solid or tangible surfaces in the environment 104.

In one embodiment, the system 100 can include one or more controllers108 that control operation of the light source(s) 102. The controller108 can represent hardware circuitry that includes and/or is connectedwith one or more processors (e.g., microcontrollers, microprocessors,field programmable gate arrays, integrated circuits, or the like) thatcontrol activation or deactivation of the light sources 102. Thecontroller 108 can direct power and/or control signals to the lightsources 102 (or drivers of the light sources 102) to control the lightsources 102. In one aspect, the controller 108 may cause the lightsources 102 to generate light of different wavelengths at differenttimes. For example, the controller 108 may direct one or more of thelight sources 102 to pulse the inactivating portion of the light at afrequency exceeding about 100 hertz (e.g., at least 105 hertz, at least100 hertz, at least 90 hertz, at least 95 hertz, etc.), with a dutyfactor of less than about 0.5 (e.g., less than 0.6, less than 0.5, lessthan 0.4, etc.), or with a duty factor of less than about 0.1 (e.g.,less than 0.2, less than 0.1, less than 0.05, etc.).

The pathogens that may be inactivated may include a variety of bacteria,such as staphylococcus (which may include, by way of non-limitingexample, methicillin-resistant staphylococcus aureus, or MRSA, oranother type of staphylococcus), clostridium difficile, streptococcus,bacterial pneumonia, etc., as well as some forms of spores, fungi, andviruses. The pathogens may be inactivated by killing the pathogens,rendering the pathogens unable to grow or reproduce, or generallyrendering them unable to cause disease in humans.

The light source(s) 102 can generate the disinfecting light within adesignated flux density range. This flux density range or power densityrange can be between several milliwatts per square meter (mW/m²), (e.g.,five mW/m² or 40 mW/m²) and several watts per square meter (e.g., two orthree or ten W/m²). In one embodiment, the flux density range extendsupward to no more than 10 W/m². Alternatively, the flux density rangecan be in another range. The light generated by the light source(s) 102may have several different wavelengths, with a portion of the lightbeing inactivating light having wavelengths that inactivate thepathogens and one or more other portions of the light having other,different wavelengths. This inactivating portion of the light may beinvisible to human beings. For example, the inactivating portion of thelight may have a wavelength that is no longer than 380 nm. As describedbelow, the inactivating portion of the light may have a lower limit onthe wavelength of light to avoid exposing human beings to hazardousradiation. For example, the inactivating portion of the light may have awavelength that is at least 280 nm, or at least 300 nm, or at least 320nm, or another lower limit.

At shorter wavelengths of light (e.g. UVB and UVC) used by otherlighting systems, pathogens may be lethally inactivated by permanentlybreaking deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)macromolecules of the pathogens. At the longer wavelengths (e.g. UVA),the inactivating portion of the light generated by the light source(s)102 may inactivate the pathogens by catalyzing chemical surfacereactions on exterior surfaces such as the cell membrane; or by causingsingle-strand DNA breaks which may be non-lethal; or by damaging lessrobust intracellular structures of the bacteria, resulting in sublethaleffects such as growth reduction, reduction of bacterial capacity forphage development, inhibition of induced-enzyme (tryptophanase)synthesis, inhibition of membrane transport, and other non-lethal damagewhich may be repaired by the cell at low doses, but which accumulate athigher doses resulting in damage levels that inactivate the cell orsignificantly reduce the growth of the cell so as to effectivelyinactivate the cell. There exist a large number of potential targetchromophores (the part of a given molecule in the cell that absorbscertain wavelengths of light and transmits or reflects others) in anygiven pathogenic cell, each chromophore potentially having a specificrange of absorbing wavelengths which may differ from other chromophores,and resulting in different photo-products due to the energy exchangefrom the chromophore to other molecules in the cell, resulting indisruption of the normal chemical processes in the cell. Whereas, thebreaking of DNA molecules by high-energy UVC is known to have a strongresonance between about 240 nm and 280 nm; and whereas it is believedthat at about 405 nm, the photoexcitation of naturally occurringendogenous porphyrins on the surface of the cell, which act asendogenous photosensitizers within the cells leads to energy transferand, ultimately, the production of highly cytotoxic, oxygen-derivedspecies, leading to cell damage or death; the mechanisms of cell damagein the UVA range is believed to be many and varied, not represented byany single resonance band of wavelengths. In this wavelength regime,having a large number of potential chromophores and resonance wavelengthbands in any given cell, it may be reasonable to hypothesize that therate at which the pathogens are inactivated by the inactivating portionof the light generated by the light source(s) 102 (which also can bereferred to as a kill or inactivation rate) may be significantly fasterfor higher energy photons than for lower energy photons. The rate atwhich the pathogens are inactivated by UVA radiation may be hypothesizedto be based on a first-order kinetic relationship, or equivalently, theArrhenius equation. The inactivation rate of the inactivating portion ofthe light may be hypothesized to increase exponentially as the photonenergy of the inactivating portion of the light increases, with thephoton energy being inversely related to the wavelength of light. As aresult, the inactivation rate of the inactivating portion of the lightmay exponentially increase as the wavelength of the inactivating portionof the light decreases.

A first-order kinetic model for the inactivation rate as a function ofphoton energy may be hypothesized by analogy to the first-order kineticmodel for inactivation as a function of temperature used in the foodprocessing industry as provided by Equation 5 in the reference titled“Safe Practices for Food Processes>Kinetics of Microbial Inactivationfor Alternative Food Processing Technologies Overarching Principles:Kinetics and Pathogens of Concern for All Technologies, published by theU.S. Food and Drug Administration, last updated on Apr. 9, 2013, hereinincorporated by reference, and referred to as 2013 FDA. In this analogy,the temperature variable in the 2013 FDA model is replaced with photonenergy for the hypothesis herein that relates inactivation rate or doseto photon energy. In 2013 FDA, the influence of temperature on microbialpopulation inactivation rates has been expressed in terms of the thermalresistance constant z(T) using the following model:

log₁₀ [D/D _(R)]=−(T−T _(R))/z(T)   Equation 5 in 2013 FDA

where D is the decimal reduction time, or time required for a 1-log₁₀(10×) cycle reduction in the microbial population; the thermalresistance constant z(T) is the temperature increase needed toaccomplish a 1-log₁₀ cycle reduction in D; the reference decimalreduction time D_(R) is the magnitude at a reference temperature TRwithin the range of temperatures used to generate experimental data. Theanalogy made herein is represented in Table 1.

TABLE 1 Variable in the Model 2013 FDA Model One Model Of The InventiveSubject Matter Dose of inactivation temperature * exposure time photonenergy * photon flux * exposure time energy to the (T*t) (E*Φ*t)pathogen population i.e., time at temperature i.e., time at irradiance Dtime at temperature required for time at irradiance required for 2-log₁₀(100x) 1-log₁₀ (10x) reduction in reduction in pathogen population atthe energy pathogen population at the test E of the test photontemperature T D_(R) time at temperature required for time at irradiancerequired for 2-log₁₀ (100x) 1-log₁₀ (10x) reduction in reduction inpathogen population at the energy pathogen population at the E of thereference photon reference temperature T_(R) Test Energy T, testtemperature E, energy of the test photon Reference Energy T_(R),reference temperature E_(R), energy of the reference photonCharacteristic Energy z(T), increase in temperature z(E), increase inthe energy of the disinfecting needed to accomplish a 1-log₁₀ photonsneeded to accomplish a 1-log₁₀ reduction in D reduction in D

The dose vs. wavelength obtained by hypothesizing a first-order kineticmodel is given by:

$\begin{matrix}{{{\log_{10}\left( {D/D_{R}} \right)} = {- \frac{E - E_{R}}{z(E)}}},} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where D is the dose required for a target (e.g. 90% or 99% or other)reduction in pathogen count using disinfecting photons of energy E;D_(R) is the dose required for the same target reduction in pathogencount using photons of reference energy E_(R); z(E) is the increase inphoton energy needed to accomplish a 1-log (90%) reduction in D. Takingthe first derivative of Equation 1 with respect to E provides

$\begin{matrix}{\frac{{\log_{10}(D)}}{E} = {{- 1}/{{z(E)}.}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

So a first-order kinetic model would be represented by a linear slopehaving value −1/z(E) in a plot of log₁₀ (dose, D) on they axis vs.photon energy E on the x axis. A steeper slope −1/z(E) indicates astronger dependence of dose of the disinfection lighting (in J/m²) vs.photon energy (in eV), at some target inactivation (e.g. 90% or 99% orother).

FIG. 2a illustrates a dataset 200, 202, 204, 206 of the dose ofdisinfecting light required for about 99% inactivation of E. colipathogens, wherein the light has a narrow spectral width (FWHM<about 20nm), with peak emission at four different wavelengths of light (254,313, 365, and 460 nm, respectively) according to the reference 1976 Webband Brown (Photochemistry and Photobiology, 24:425-432, 1976). The data200, 202, 204, 206 represent the accumulated doses at which thepathogens are inactivated by 99% (2-log₁₀) relative to the controlswhich were not dosed, where the first horizontal axis 210 represents theenergy of photons of the light (in electron volts, or eV) at thewavelength of peak emission, and the second horizontal axis 212represents the wavelengths of the inactivating portions of the light (innm) at the emission peak, and a vertical axis 214 represents the commonlogarithm (also known as the decimal log, or base-10 log, or log₁₀) ofthe dose of disinfecting light required for about 99% inactivation (interms of Joules per square meter, J/m²). A linear trend line fit 220 tothe subset of the data 200, 202, and 204; and a linear trend line fit222 to the subset of the data 202, 204, and 206 also are shown. Thetrend line 220 represents a logarithmic relationship between the photonenergy and the dose of disinfecting light, having a slope of−1/z(E)=−3.3 log₁₀ per eV of photon energy in the range of about 3.4 toabout 4.9 eV, corresponding to wavelengths from about 254 to about 365nm. The trend line 222 represents a logarithmic relationship between thephoton energy and the dose of disinfecting light, having a slope of−1/z(E)=−2.3 log₁₀ per eV of photon energy in the range of about 2.7 toabout 3.9 eV, corresponding to wavelengths from about 313 to about 460nm. It would be expected that the trend line slope that applies in therange of about 405 to about 365 nm (i.e., the range between a typicalviolet-light disinfection system having peak emission at about 405 nm,and an embodiment of the present invention having peak emission at about365 nm) should be bracketed by the slopes of the two trend lines in FIG.2a (i.e., −1/z(E)=−3.3 log₁₀ and −1/z(E)=−2.3 log₁₀ per eV). It isreasonable to expect that the trend line slope that applies in the rangeof about 405 to about 365 nm is about equal to the average of those twoslopes, or about −1/z(E)=−2.8 log₁₀ per eV for 99% inactivation of E.coli. Given a slope of −1/z(E)=2.8 log₁₀ per eV, the ratio of doserequired for 99% inactivation of E. coli using narrow-band light havingpeak emissions at about 365 nm (3.40 eV) and about 405 nm (3.06 eV) is10̂2.8*(3.40−3.06)=8.7. From FIG. 2a , it is expected that the dose ofdisinfecting light required for about 99% inactivation of E. coli may bereduced by about 8.7× when the wavelength is reduced from about 405 nmto about 365 nm. A greater (lesser) ratio would be expected atwavelengths shorter (longer) than about 365 nm, compared with about 405nm.

FIG. 2b illustrates a dataset 230, 232, 234 of the dose of disinfectinglight required for about 99% inactivation of S. aureus pathogens,wherein the disinfecting light is provided by LEDs having a narrowspectral width (FWHM<about 20 nm), with peak emission at three differentwavelengths of light (369, 388, and 404 nm, respectively) according tothe present invention. A linear trend line fit 240 to the data 230, 232,and 234 also is shown. The data 230, 232, 234 represent the accumulateddose at which the pathogens are inactivated by 99% (2-log₁₀) relative tothe controls which were not dosed, where the first horizontal axis 210represents the energy of photons of light, and the second horizontalaxis 212 represents the wavelengths of the inactivating portions of thelight (in nm), and a vertical axis 214 represents the lop) of theaccumulated dose of disinfecting light (in J/m²). The inactivation trendline 240 represents a logarithmic relationship between the photon energyand the dose, having a slope of −1/z(E)=−3.7 lop) per eV of photonenergy in the range of about 3.0 to about 3.4 eV, corresponding towavelengths from about 369 to about 404 nm. These data were obtainedusing commercially available LEDs having measured peak wavelengths of369, 388, and 404 nm, which were labeled by the manufacturers as havingnominal peaks at 365, 390, and 405 nm, respectively. This variancebetween measured and labeled wavelength is typical for commerciallyavailable LEDs which are typically binned within +/−5 nm ranges. Itwould be expected that the trend line slope that applies in the range ofabout 405 to about 365 nm (i.e., the range between a typicalviolet-light disinfection system with peak emission at about 405 nm, andan embodiment of the present invention with peak at about 365 nm) shouldbe bracketed by the slopes of the two trend lines in FIG. 2a (i.e.,−1/z(E)=−3.3 log₁₀ per eV and −1/z(E)=−2.3 log₁₀ per eV; the average ofthe two slopes is about −1/z(E)=−2.8 log₁₀ per eV). In fact, the trendline 240 fitting the data of the present invention is about −1/z(E)=−3.7log₁₀ per eV, exceeding the expected range of slopes from the prior artdata in FIG. 2a . Given a slope of −1/z(E)=−3.7 lop) per eV, the ratioof dose required for 99% inactivation of S. aureus using narrow-bandlight having peak emissions at about 405 nm (3.06 eV) relative to doserequired at about 365 nm (3.40 eV) is 10̂3.7*(3.40−3.06)=17.4. From FIG.2b , it is expected that the dose of disinfecting light required forabout 99% inactivation of S. aureus may be reduced by about 17.4× whenthe wavelength is reduced from about 405 nm to about 365 nm. A greater(lesser) ratio would be expected at wavelengths shorter (longer) thanabout 365 nm, when compared with about 405 nm. This beneficial ratio ofdoses at 365 nm vs. 405 nm having a value of about 17.4 is unexpectedly,significantly greater than the 8.7 ratio of doses anticipated from theprior art. Even though the data in FIG. 2a pertain to E. coli, whilethat in FIG. 2b pertains to S. aureus, it is the wavelength dependenceof the required dose that we are investigating, not the relative doserequired between the two different pathogens.

FIG. 2c illustrates the dataset 230, 232, 234, including the lineartrend line fit 240, from FIG. 2b , along with the four lines 250, 252,254, 256 representing the dose vs. wavelength obtained from thefirst-order kinetic model of Equation 1, corresponding to characteristicenergies, −1/z(E)=−3.7, −3.3, −2.8, −2.3 log₁₀ per eV, respectively. Theline 250 represents the dose vs. energy relationship expected fromEquation 1 where −1/z(E)=−3.7 log₁₀ per eV corresponding to the trendline fit to the data 230, 232, 234 of the present invention; the line252 represents the dose vs. energy relationship expected from Equation 1where −1/z(E)=−3.3 log₁₀ per eV corresponding to the trend line fit tothe data 200, 202, 204 of the reference 1976 Webb and Brown; the line256 represents the dose vs. energy relationship expected from Equation 1where −1/z(E)=−2.3 log₁₀ per eV corresponding to the trend line fit tothe data 202, 204, 206 of the reference 1976 Webb and Brown; the line254 represents the dose vs. energy relationship expected from Equation 1where −1/z(E)=−2.8 log₁₀ per eV corresponding to the average of thetrend line fit to the data 200, 202, 204, and the trend line fit to thedata 202, 204, 206 of the reference 1976 Webb and Brown. The line 254represents the most likely trend line pertaining to the wavelength rangefrom about 405 nm to about 365 nm for the data 200, 202, 204, 206 fromthe reference 1976 Webb and Brown. FIG. 2d illustrates the dataset 230,232, 234, including the linear trend line fit 240, from FIG. 2 b; alongwith the four lines 250, 252, 254, 256 from FIG. 2c , representing thedose vs. wavelength obtained from the first-order kinetic model ofEquation 1, corresponding to trendline fits, −1/z(E)=−3.7, −3.3, −2.8,−2.3 log₁₀ per eV, respectively; along with a plot 260 of the maximumallowed dose vs. photon energy according to the Actinic UVphotobiological hazard function, normalized to match the dose 234 atabout 400 nm, having a first slope 262 of −1.7 log₁₀ per eV in the rangefrom about 3.1 eV to about 3.8 eV (about 400 nm to about 325 nm) andhaving a second slope 264 of −9.2 log₁₀ per eV in the range from about3.8 eV to about 4.1 eV (about 325 nm to about 300 nm). Since the ActinicUV hazard function 260 is normalized to the dose 234 at 400 nm, and theslopes of each of the first-order kinetic model curves 250, 252, 254,256 exceed the slope 262 (−1.7 log₁₀ per eV) of the curve 260 in therange 325 to 400 nm, then the safety margin between the dose requiredfor 99% disinfection of S. aureus and the maximum dose permitted by theActinic UV hazard function increases as the wavelength of thedisinfecting light decreases from 400 nm to 325 nm. This increasingmargin of safety vs. decreasing wavelength at wavelengths below 400 nmis the opposite trend that is expected from the prior art. Atwavelengths shorter than about 325 nm, the slope 264 of the Actinic UVhazard function 260 increases sharply to about −9.2 log₁₀ per eV,greatly exceeding the slopes of each of the first-order kinetic modelcurves 250, 252, 254, 256, so that the margin of safety vs. decreasingwavelength at wavelengths below about 325 nm diminishes rapidly. Therelative margin of safety becomes one at the intersection of the ActinicUV hazard function 260 with each of the first-order kinetic model curves250, 252, 254, 256 at the circles 270, 272, 274, 276, respectively,corresponding to photon energies of about 4.12, 4.05, 4.00, 3.95 eV,respectively, and wavelengths of about 301, 306, 310, 314 nm,respectively. This relatively narrow range of intersection wavelengthsin the range from about 301 nm to about 314 nm, or approximately 300 nmto 315 nm, emphasizes that the actual value of the slope of thefirst-order kinetic model is not critical, as long as it exceeds theslope 262 (−1.7 log₁₀ per eV) of the Actinic UV hazard function 260 inthe range 325 nm to 400 nm. So, if the first-order kinetic model for 99%disinfection of S. aureus has a slope that is steeper than −1.7 log₁₀per eV, then the safety margin will increase down to about 325 nm, andpossibly a somewhat shorter wavelength. The slope 264 (−9.2 log₁₀ pereV) of the Actinic UV hazard function 260 increases so sharply at lessthan 325 nm, that the intersection wavelength occurs at wavelengths onlyslightly less than 325 nm for any kinetic model having a slope much lesssteep than the slope 264 (−9.2 log₁₀ per eV) of the Actinic UV hazardfunction 260 in the range 300 to 325 nm. At any wavelength having asafety margin >1 (i.e., in the range from 400 nm down to about 300 to315 nm), the dose of disinfecting light required for 99% inactivation ofS. aureus is safer for humans than the dose of disinfecting light at 400nm that is required for 99% inactivation of S. aureus. This is contraryto the expectations proposed in the prior art. For example, the dose ofdisinfecting light required for 99% inactivation of S. aureus at about365 nm is about 0.67-log10 (about 4.7×) safer for humans than the doseof disinfecting light at 400 nm, using the slope 250 (−3.7 log₁₀ per eV)of the model that is the best fit to the data 230, 232, 234 of thepresent invention. Using the more conservative slope 256 (−2.3 log₁₀ pereV) of reference 1976 Webb and Brown, the dose of disinfecting lightrequired for 99% inactivation of E. coli at about 365 nm is about0.37-log10 (about 2.4×) safer for humans than the dose of disinfectinglight at 400 nm. Similar trends and conclusions may be expected fordisinfection levels exceeding 90-99%, as well. The safety marginsincrease vs. decreasing wavelengths down to about 325 nm, and thendecline to <1 in the range of about 300 to 315 nm, as shown in Table 2.

TABLE 2 Wavelength of Safety margin for 99% Safety margin for 99%disinfecting disinfection of S. aureus disinfection of E. coli light[nm] @ −3.7 log₁₀ per eV @ −2.3 log₁₀ per eV 405 1.0 1.0 385 2.1 1.5 3654.7 2.4 345 12 3.9 325 29 6.5 305 1.6 <1

As shown above, reducing the wavelength of the inactivating portion ofthe light below about 400 nm, and especially below about 380 nm, isunexpectedly safer with regard to the Actinic UV photobiological hazardthan disinfecting pathogens with light at about 400 nm, or in.Considering the other two of the three hazards that pertain to lightsources emitting in the blue, violet, and UV ranges, FIG. 3 illustratesall three hazard functions: the Actinic UV 310, Near UV 320, and BlueLight 330 hazards. The log₁₀ (Actinic UV) curve 312 is also illustratedin FIG. 3. The hazard functions 310, 320, 330 are shown alongside ahorizontal axis 300 representative of wavelengths of light and avertical axis 302 representative of the risk posed by exposure to thelight as a function of the wavelength of the light source. The ActinicUV hazard has been shown above to be unexpectedly less hazardous forwavelengths in the range from about 315 nm to about 380 nm than forlonger wavelengths in the range from about 380 nm to about 400 nm atdoses required to inactivate pathogens at the 90-99% level. Theadditional two photobiological hazards are, as expected from thespectral shapes of the hazard functions, not any more hazardous in the315-380 nm range than in the 380-400 nm range at doses required toinactivate pathogens at the 90-99% level. The hazard function 312 isshown alongside a horizontal axis 300 representative of wavelengths oflight and a second vertical axis 304 representing the common logarithm(log₁₀) of the Actinic UV hazard function. Larger values along thevertical axis 304 represent greater health hazards than smaller values.All hazard values are normalized to 1.0. The maximum allowable emissionfrom a light source, with regard to each photobiological hazard isdetermined from the convolution integral of the spectral powerdistribution (SPD) of irradiance of the light source (in W/m²-nm) withthe hazard function, integrated over all wavelengths. The calculation istypically performed as a sum-product of the SPD and the hazard function,with each factor discretized in 1 nm or 2 nm or 5 nm increments. Herein,the sum-products are calculated with 1 nm discretization. The integralor sum-product must be less than the maximum allowable limit for eachhazard. The allowable limits for Exempt and Low Risk light sources asspecified by IEC 62471 are shown in Table 3a:

TABLE 3a Hazard Exempt Limit Low Risk Limit Actinic 30 J/m² within any30,000 30 J/m² within any 10,000 UV second period (equivalent secondperiod (equivalent to 0.001 W/m²) to 0.003 W/m²) UVA 10 W/m² for expo-33 W/m² for expo- sures >1000 seconds sures >300 seconds Blue 100 W/m²srfor expo- 10,000 W/m²sr for expo- Light sures >10,000 seconds sures >100seconds

The hazard function 330 can be referred to as a blue light hazardfunction, as it represents the risk to humans posed by exposure towavelengths of blue (and violet, and some UV) light. The riskrepresented by the hazard function 330 is maximum (1.0 value) between435 nm and 440 nm; is very high (>0.5 value) between about 410 nm andabout 480 nm; is low (<0.1 value) below 400 nm and above 500 nm; and isextremely low (0.01 value) between 300 nm and 380 nm. Therefore, theblue light hazard which poses significant risk in prior art disinfectionsystems operating in the range of about 380 nm to about 420 nm, isdiminishingly small at wavelengths below about 380 nm. The hazardfunction 320 can be referred to as a UVA hazard function, since itrepresents risks to humans posed by exposure to wavelengths of light inthe UVA band. The hazard function 320 is flat (1.0 value) between 315 nmand 400 nm, and zero outside that range. Other disinfection lightingsystems operating at about 405 nm having a FWHM of about 10 nm emitsabout 10% of its light within the UVA range, and so is typically safe atemission levels sufficient for disinfecting pathogens. The disinfectionlighting system of the present invention, with peak emission in the UVrange from about 300 nm to about 380 nm, could be unsafe with regard tothe Near UV hazard if the emission in the UV range is made as strong asthe emission of the prior art disinfection lighting systems having peakemission at about 405 nm, but since the slope 240 (−1/z(E)=3.7 log₁₀ pereV) corresponding to the trend line fit to the data 230, 232, 234 of thepresent invention is so steep, where the Near UV hazard function isflat, that any wavelength below about 380 nm will provide sufficientdisinfection lighting without exceeding the Near UV hazard limit.Overall, it has been found, unexpectedly, that the dose of disinfectionlight required to inactivate about 90-99% of pathogens using narrow-bandlight having peak emission in the range from about 300 nm to about 380nm has margins of safety relative to the three photobiological hazardspertaining to this part of the spectrum that are comparable to, or saferthan, the margins of safety for 90-99% disinfection of pathogens usingnarrow-band light having peak emission in the range of about 380 nm to420 nm. The safety margins provided by the prior art and by thisinvention are summarized in Tables 3b and 3c for a dose of disinfectionlight sufficient to provide 90% and 99% kill, respectively, of S. aureusover a period of 8 hours. The safety factors presented in tables 3b and3c are relative to the Exempt hazard limit. Safety factors would behigher in all cases relative to the Low Risk limit.

TABLE 3b 90% disinfection of S. aureus Inventive Inventive Other SystemsSubject Matter Subject Matter Peak at 405 nm Peak at 365 nm Peak at 325nm 55 J/cm² 11 J/cm² 0.3 J/cm² Exempt Re- Mar- Re- Mar- Re- Mar- HazardLimit sult gin sult gin sult gin Actinic 0.001 0.0001 10 0.0004 2.50.00018 5.4 UV W/m² UVA 10 2.9 3.4 3.7 2.7 0.10 87 W/m² Blue Light 10087 1.2 4.3 23 3.6 28 W/m²sr Du′v′ 0.236 0.003 0.000

TABLE 3c 99% disinfection of S. aureus Inventive Inventive Prior artSubject Matter Subject Matter Peak at 405 nm Peak at 365 nm Peak at 325nm 109 J/cm² 21.4 J/cm² 0.7 J/cm² Exempt Re- Mar- Re- Mar- Re- Mar-Hazard Limit sult gin sult gin sult gin Actinic 0.001 0.00020 5.10.00080 1.2 0.00037 2.7 UV W/m² UVA 10 5.8 1.7 7.4 1.4 0.23 43.8 W/m²Blue Light 100 170 0.6 5 19.3 3.6 27.8 W/m²sr Du′v′ 0.317 0.006 0.000

Table 3b indicates that disinfection lighting sufficient to provideabout 90% inactivation of S. aureus is safe relative to all three of therelevant photobiological hazards for 405, 365, and 325 nm. It shows thatwhile the 405 nm disinfection lighting is only marginally safe relativeto the Blue Light Hazard, the 325 nm and 365 nm disinfection lightinghave high safety margins relative to all three hazards.

Table 3c indicates that disinfection lighting sufficient to provideabout 99% inactivation of S. aureus is safe relative to all three of therelevant photobiological hazards only for 325 nm and 365 nm. It showsthat the 405 nm disinfection lighting can become unsafe relative to theBlue Light Hazard, if higher disinfection levels are desired.

The safety factor for the Actinic UV, UVA, and Blue Light Hazard areshown for a range of wavelengths in FIGS. 9a -9 c. Line 901 in FIG. 9arepresents the safety factor for the Actinic UV hazard function over arange of 250-420 nm. Values of 1 or greater represent that a lightdisinfection system would be able to inactivate 90% of pathogens in 8hours while being safe for a human. The effectiveness of theinactivating portion of the light at a given wavelength is determined byEquation 1 with the slope −1/z(E)=−2.8 log₁₀ per eV. Line 902 and 903represent the safety factors for UVA and Blue Light hazards,respectively. Line 904 in FIG. 9b represents the minimum safety factorat each wavelength (the minimum of the Actinic, UVA, and Blue Lightsafety factors represented by lines 901, 902, and 903 in FIG. 9a ). Thisshows that the 405 nm inactivating light of the prior art is safe forthis pathogen inactivation rate, but that 400 nm and 410 nm inactivatingis not safe. Because commercially produced LEDs have some variance intheir peak wavelength emission, the use of 405 nm LEDs must be tightlycontrolled in practice, so as to avoid using LEDs at slightly longer orshorter wavelengths that may be hazardous to humans. Line 904 also showsthat the present invention allows the safe use of inactivating light inthe range of 320-370 nm at this pathogen kill rate. This wide range ofacceptable wavelengths allows for the use of commercially produced andbinned LEDs without requiring that the peak wavelength of their emissionbe tightly controlled. Lines 905, 906, and 907 in FIG. 9c showadditional curves for minimum safety factor for different values of−1/z(E) (−3.7, −3.3, and −2.3 log₁₀ /eV, respectively), along with Line904 from FIG. 9b which represents −1/z(E)=−2.8 log₁₀ /eV. Line 905 showsa local maximum safety factor of 5.4 at 325 nm, with a safe range of310-380 nm. Line 906 shows a local maximum safety factor of 3.4 at 330nm, with a safe range of 315-375 nm. Line 904 shows a local maximumsafety factor of 1.7 at 355 nm, with a safe range of 320-370 nm. Line907 shows a local maximum safety factor of 1.1 at 355 nm, with a saferange of 350-365 nm. Additionally, lines 905, 906, and 904 show a safearea at low wavelengths (less than about 280, 270, or 260 nmrespectively). This indicates that due to the increased pathogeninactivation ability shown by the kinetic model, these UV-C wavelengthsmay be able to achieve 90% pathogen inactivation over 8 hours ofexposure while being safe for humans.

Also indicated in Tables 3b and 3c are the shifts in color point (Du′v′)in the International Commission on Illumination (CIE) 1976 (u′v′)chromaticity diagram. The maximum allowable color shift or colordifference that is specified by customers in many typical LED lightingsystems is Du′v′ 0.007 or <0.005, and sometimes <0.002. The Du′v′ valuesindicated in Tables 3b and 3c pertain to the flux of disinfectionlighting required to inactivate 90% or 99% of S. aureus, respectively,added to and mixed with a flux of typical white lighting (e.g. 4000 K,80 CRI, on the blackbody locus) at an illuminance of 500 lux(lumens/m²), which is a typical indoor illuminance. Values of Du′v′exceeding about 0.007 or about 0.005 or about 0.002 indicate that theillumination provided by the mixture of the white light with thedisinfecting light are shifted too far away from the target color pointof the white light to be acceptable in most customer applications,requiring a correction to the color point by addition of a thirdcomponent of light to offset the color shift created by the disinfectioncomponent of the light. Tables 3b and 3c indicate that Du′v′ does notexceed 0.006 for either 365 nm or 325 nm disinfection lighting, ateither 90% or 99% disinfection levels, but that 405 nm disinfectionlighting exceeds even the most relaxed limit of Du′v′<0.007 by more than40×, so that the color appearance of the mixed light is so far away fromthe color point specification, that an extreme amount of colorcorrection is required from the third component of the light whichsignificantly increases the complexity, the color stability, andpotentially the cost of the lighting system.

The extreme color distortion that is indicated by Du′v′ values of 0.236or 0.317 when using 405 nm light to achieve 90% or 99% inactivation ofS. aureus are indicative of the extremely unusual appearance of themixture of disinfection light with standard white light, whenuncorrected by the third component of light. However, even though thethird component of light may mask the distortion of the white lightcaused by the extreme amount of 405 nm disinfection light, and eventhough it may correct the color point of the overall lighting system, itdoes not reduce the extremely high flux of 405 nm light, which is stillreceived, unabated, by the retina of the human subject. Such extremelyhigh levels of blue or violet or near UV light are well known to causephysiological disturbances including headache, dizziness, nausea, andothers. Those adverse side effects of the disinfection component of thelighting have not appeared in our testing of subjects using 365 nm lightat doses high enough to provide 90% or 99% inactivation of S. aureus.The reasons why 365 nm and other UV wavelengths may avoid the adversephysiological reactions of human subjects is because those wavelengthsare nearly imperceptible to the human eye, and because the flux ofdisinfection light required for 90-99% inactivation of pathogens is muchlower below about 380 nm than in the visible wavelength ranges,including the 380-400 nm range, and longer wavelength ranges.

The inactivation rates 610, 612, 614, 616, 618, 620 in FIG. 6 representthe rates at which the pathogen staphylococcus aureus was inactivated byexposure of narrow-band light having peak emission wavelengths at 400,405, 410, 415, 420, 425 nm, respectively, as described in the Maclean2008 reference. The inactivation rate vs. wavelength is shown to have apeak 612 at 405 nm, which is a typical preferred peak emissionwavelength in some known systems. It is proposed in the U.S. Pat. Nos.8,398,264 and 9,039,966 that the peak at about 405 nm is due to resonantabsorption of the photon by a porphyrin molecule resident on the surfaceof the pathogen, resulting in generation of reactive oxygen speciesleading to inactivation of the pathogen. Herein, this resonanceabsorption can be referred to as the Porphyrin Hypothesis. The PorphyrinHypothesis states that the ideal wavelength range for inactivation ofpathogens without exceeding photobiological hazard limits may be between380 nm and 420 nm, and especially at or near 405 nm. The data andcalculations provided herein demonstrate that a different hypothesis,the First-order Kinetic Hypothesis, has guided discovery of greatlyenhanced inactivation rates at wavelengths below about 380 nm, whichunexpectedly resolve the major shortcomings of the prior art, includingproviding higher electrical system efficiency; lower system cost; lessdistortion of the color point when mixed with white light; reduced oreliminated physiological disturbance to humans; greater photobiologicalsafety for humans; higher inactivation rate of pathogens. Although oneor more embodiments of the system 100 described herein includes lightsource(s) 102 that generate light having wavelengths that extend intothe hazard functions 310, 320, 330 the light is generated at asufficiently low power that the risk to human beings exposed to thelight is still sufficiently low to avoid harming the human beings. Forexample, the limit of exposure as specified by IEC 62471 to the bluelight hazard function 330 is 100 watts per steradian per square meter.The light generated by the light source(s) 102 that falls within theblue light hazard function 300 may have a power density of no more than100 watts per steradian per square meter, such as 100 watts or 5 wattsper steradian per square meter or less.

As another example, the limit of exposure as specified by IEC 62471 tothe UVA hazard function 320 may be 10 watts per square meter. The lightgenerated by the light source(s) 102 that falls within the UVA hazardfunction 320 may have a power density of less than 10 watts per squaremeter, such as 4 watts or 0.5 watts per square meter or less. Forexample, the light source may generate the inactivating portion of thelight such that the light includes no more than 10 watts or no more than4 watts 0.5 watts of ultraviolet A light (e.g., the wavelengths of lightfalling within the range of the UVA hazard function 302) per squaremeter of floor area in the environment 104.

As another example, the limit of exposure as specified by IEC 62471 tothe actinic hazard function 310 may be 0.001 watts per square meter. Thelight generated by the light source(s) 102 that falls within the actinichazard function 310 may have a power density of less than 0.001 wattsper square meter, such as 0.0005 watts or 0.00015 watts per square meteror less. For example, the light source may generate the inactivatingportion of the light such that the light includes no more than 0.001watts or no more than 0.0005 watts or no more than 0.00015 watts ofactinic ultraviolet light (e.g., the wavelengths of light falling withinthe range of the actinic hazard function 304) per square meter of floorarea in the environment 104.

Based on the preceding inactivation rates, power densities, and hazardfunctions, the lighting system 100 shown in FIG. 1 can generate lightfrom the light sources 102 having an inactivating portion thatinactivates pathogens on the surfaces or materials 106 in theenvironment 104 (e.g., without using a photosensitizer), where theinactivating portion of the light has a peak wavelength of no more than380 nanometers and a flux density or power density of no more than 20watts or 10 watts or 0.5 watts per square meter (e.g., of surface areaor area on the floor of the environment 104) in order to ensure thesafety of human beings exposed to the light. Such a light has been foundto successfully inactivate a substantial amount of pathogens with arapid inactivation rate.

In one example of the inventive subject matter described herein,irradiance tests were performed using a clinical wound-isolate ofStaphylococcus aureus (ATCC #29213). Bacteria were inoculated andcultured overnight in tryptic soy broth (TSB) to high density at 37° C.with shaking. Before each experiment, overnight cultures were dilutedback to log phase in fresh TSB and grown for approximately 2 hours at37° C. with shaking. After 2 hours of re-culture, culture density wasmeasured by optical density at 600 nm (0D600) and cell counts wereestimated from a 0.5 McFarland standard (A600 of 0.132≅1.5×10⁸ CFU/mL,where CFU denotes Colony-Forming Units). Table 4 below outlines the celldensity and serial dilution methodology for three test conditions.

TABLE 4 Estimated Final Test Initial Culture Culture Density conditiondensity (CFU/mL) First Dilution Second Dilution (CFU/mL) A 25.51 × 10⁸ 5μL into 5 mL 178 μL into 19.8 mL ~2 × 10⁴ (TSB) (TSB) B 31.25 × 10⁸ 5 μLinto 5 mL 128 μL into 19.8 mL ~2 × 10⁴ (0.9% saline) (0.9% saline) C15.17 × 10⁸ 5 μL into 5 mL 264 μL into 19.7 mL ~2 × 10⁴ (TSB) (0.9%saline)

Approximately 5 mLs of diluted bacteria (Second Serial Dilution stock,˜2×10⁴ CFU/mL) were transferred into Falcon Easy-Grip Tissue Culturepolystyrene dishes (#353004, Corning Life Sciences). For all conditions,the lids of the Petri dishes were removed during light irradiation.Petri dishes were placed inside a steel housing to block ambient outsidelight. LED lamps were mounted onto the steel housings to irradiate thePetri dish test samples from a separation distance of 4 inches. Allirradiation experiments were conducted inside a Biosafety Cabinet with astainless steel working surface. Control samples were incubated in thedark under a lamp head with no connected power.

Petri dish test-samples were exposed to LED light having peakwavelengths of 404 nm or 369 nm for 4 hours and bacteria were platedonto solid TSB agar for standard Colony-Forming Unit (CFU) analysis. Theculture was mixed and diluted into either sterile TSB or 0.9% saline by10-fold serial dilution. Aliquots of 1004 were pipetted onto solid TSBagar plates (in duplicate) and spread using glass beads. Plates wereincubated for 12-24 hours at 37° C. and resulting colonies wereenumerated. Viable cell density (per mL) was calculated by multiplyingthe number of colonies (per plate) by a 10-fold plating dilution and anyappropriate serial-dilution factors thereafter. Table 5 below summarizesthe growth results of these experiments at 404 nm and 369 nm for each ofthe tested conditions described above. In some cases, bacterial growthon the plate was too numerous to count (TNTC). The results from controlsamples generally agreed with the estimated inoculum of approximately2×10⁴ CFU/mL.

TABLE 5 Test Light Non-diluted 10-fold diluted 100-fold diluted AverageCondition Exposure colony count colony count colony count CFU/mL AControl TNTC TNTC Plate 1: 38 5.35 × 10⁴ Plate 2: 69 A 369 nm Plate 1: 0Plate 1: 0 Plate 1: 0 0 Plate 2: 0 Plate 2: 0 Plate 2: 0 A 404 nm Plate1: 0 Plate 1: 0 Plate 1: 0 0 Plate 2: 0 Plate 2: 0 Plate 2: 0 B ControlTNTC Plate 1: 150 NA 1.25 × 10⁴ Plate 2: 100 B 369 nm Plate 1: 0 Plate1: 0 NA 0 Plate 2: 0 Plate 2: 0 B 404 nm TNTC Plate 1: 79 NA 6.45 × 10³Plate 2: 50 C Control TNTC Plate 1: 150 Plate 1: 11 1.41 × 10⁴ ± Plate2: 125 Plate 2: 18 3.07 × 10³ C 369 nm Plate 1: 6 Plate 1: 0 Plate 1: 07.75 × 10¹ ± Plate 2: 5 Plate 2: 2 Plate 2: 0 8.58 × 10¹ C 404 nm Plate1: 130 Plate 1: 11 Plate 1: 1 1.28 × 10³ ± Plate 2: 125 Plate 2: 20Plate 2: 1 3.76 × 10²

The results of these experiments demonstrate several novel findings.First, by comparing conditions A-C, it is revealed that theantibacterial activity of 404 nm light is potentiated by mediacomponents in rich media (i.e., condition A versus condition B), evenwhen trace amounts of TSB media are carried over by dilution (i.e.,condition C versus condition B). This finding is consistent with theunderstanding that photo-inactivation at 404 nm was found to bedependent on light-sensitive components in rich media, as described inTomb et al., Inactivation of Streptomyces phage φC31 by 405 nm light,Bacteriophage 4, e32129; January-December 2014.

In contrast, the antibacterial activity of 369 nm light (e.g., theinactivating portion of the light generated by one or more of the lightsources 102 shown in FIG. 1 according to one embodiment) is generallyunbiased by media test conditions and inactivates bacteria to a greaterdegree under all test conditions (A-C). Interestingly, by comparingconditions A-C, it is suggested that light-sensitive components in richmedia might exert inverse effects at 369 nm light compared to 405 nmlight, depending on the concentration of media components.

FIG. 4 illustrates a bar graph of log-reduction at 369 nm compared to404 nm for the tested conditions A-C (normalized to untreated controls).This data generally illustrates that 369 nm light photo-inactivatesbacteria via different environmental and mechanistic parameters than 404nm light.

In another example, irradiance tests were performed using a clinicalwound-isolate of Staphylococcus aureus (ATCC #29213). Bacteria wereinoculated and cultured overnight in tryptic soy broth (TSB) to highdensity at 37° C. with shaking. Before each experiment, overnightcultures were diluted back to log phase in fresh TSB and grown forapproximately 2 hours at 37° C. with shaking. After 2 hours ofre-culture, culture density was measured by optical density at 600 nm(0D600) and cell counts were estimated relative to a 0.5 McFarlandstandard (A600 of 0.132≅1.5×10⁸ CFU/mL). Bacteria were diluted to 2×10⁴CFU/mL in isotonic saline as described in Table 4 for Condition B inorder to avoid artifacts associated with irradiance of rich media (asdescribed above in connection with the preceding example).

Approximately 5 mLs of diluted bacteria were transferred into FalconEasy-Grip Tissue Culture polystyrene dishes (#353004, Corning LifeSciences), and the lids of the Petri dishes were removed during lightirradiation. Petri dishes were placed inside a steel housing to blockambient outside light and LED lamps were mounted on the steel housingsto irradiate bacteria from a separation distance of 4 inches. Allirradiation experiments were conducted inside a Biosafety Cabinet with astainless steel working surface. Control samples were incubated in thedark under a lamp head with no connected power.

Petri dish test-samples were exposed to LED light having peakwavelengths of 404 nm, 388 nm, or 369 nm for 2-4 hours, and bacteriawere plated onto solid TSB agar for standard Colony-Forming Unit (CFU)analysis. The culture was mixed and diluted into either sterile TSB or0.9% saline by 10-fold serial dilution, and aliquots of 1004 werepipetted onto solid TSB agar plates (in duplicate) and spread usingglass beads. Plates were incubated for 12-24 hours at 37° C. andresulting colonies were enumerated. Viable cell density (per mL) wascalculated by multiplying the number of colonies (per plate) by a10-fold plating dilution and any appropriate serial-dilution factorsthereafter. Table 6 below summarizes the results of these experimentsfor the irradiance conditions described above. In some cases, bacterialgrowth on the plate was too numerous to count (TNTC). The results fromcontrol samples generally agreed with the estimated inoculum ofapproximately 2×10⁴ CFU/mL.

TABLE 6 Exposure Light Non-diluted 10-fold diluted 100-fold dilutedAverage Time Exposure colony count colony count colony count CFU/mL 2hrs Control TNTC Plate 1: 183 Plate 1: 14 1.53 × 10⁴ ± Plate 2: 139Plate 2: 15 2.06 × 10³ 2 hrs 369 nm TNTC Plate 1: 71 Plate 1: 5 5.83 ×10³ ± Plate 2: 72 Plate 2: 4 1.58 × 10³ 2 hrs 388 nm TNTC Plate 1: 137Plate 1: 9 1.16 × 10⁴ ± Plate 2: 122 Plate 2: NA 2.4 × 10³ 2 hrs 404 nmTNTC Plate 1: 128 Plate 1: 10 1.06 × 10⁴ ± Plate 2: 96 Plate 2: 10 1.48× 10³ 3 hrs Control TNTC Plate 1: 82 Plate 1: 9 8.73 × 10³ ± Plate 2: 90Plate 2: NA 4.62 × 10² 3 hrs 369 nm Plate 1: 0 Plate 1: 0 Plate 1: 0 0Plate 2: 0 Plate 2: 0 Plate 2: 0 3 hrs 388 nm Plate 1: 34 Plate 1: 4Plate 1: 0 3.48 × 10² ± Plate 2: 35 Plate 2: 3 Plate 2: 0 4.11 × 10¹ 3hrs 404 nm TNTC Plate 1: 90 Plate 1: 7 8.18 × 10³ ± Plate 2: 77 Plate 2:9 9.95 × 10² 4 hrs Control TNTC Plate 1: 100 NA 1.25 × 10⁴ Plate 2: 1504 hrs 369 nm Plate 1: 0 Plate 1: 0 NA 0 Plate 2: 0 Plate 2: 0 4 hrs 388nm Plate 1: 18 Plate 1: 3 NA 2.15 × 10² Plate 2: 28 Plate 2: 1 4 hrs 404nm TNTC Plate 1: 79 NA 6.45 × 10³ Plate 2: 50

The results of these experiments demonstrate several novel findings.First, by comparing the kinetics of photo-inactivation, it is revealedthat 369 nm light rapidly inactivates at least 4-log of staphylococcusaureus inoculant between 2-3 hours of exposure. Photo-inactivation using388 nm light is approximately 2-fold less efficient than 369 nm light,and little to no photo-inactivation is observed with 404 nm light underthese experimental parameters that minimize environmental artifacts frommedia components. FIG. 5 depicts the photo-inactivation kinetics of 369nm, 388 nm, and 404 nm light after normalization to untreated controls.

The results achieved by these experiments demonstrate that theinactivation rate for inactivating pathogens unexpectedly andsignificantly increases as the wavelength of the inactivating portion ofthe light is decreased, while the power density of the inactivatingportion of the light remains sufficiently low to be safe for humanexposure to the light. Prior attempts to inactivate pathogens usinglight rely on light that either has short wavelengths and larger powerdensities, which poses significant risks of exposure to human beings dueto the actinic health hazard 310 such that human beings cannot bepresent when a location is exposed to the light or the exposed locationis not accessible to human beings. Other prior attempts rely on a lighthaving a peak wavelength of 405 nm that predominantly lies between theUVA and blue light hazard functions 320, 330, but that also usesincreased power densities and that is visible to human observers, whichcan cause undesirable effects to the exposed human observers, such asnausea, dizziness, etc. The reduced wavelength, reduced power light usedby the lighting system 100 shown in FIG. 1 can produce light of shorterwavelengths and less power density, while being invisible to humanobservers, safe for exposure to human observers, and having aninactivation rate that is several orders of magnitude faster than theprior attempts.

A method for inactivating one or more pathogens by exposing thepathogens to light includes generating light from a light source thatexposes one or more surfaces or materials to the light, where aninactivating portion of the light has a peak wavelength in the range of300 to 380 nanometers in one embodiment.

In one embodiment, a lighting system includes a light source configuredto generate light toward one or more surfaces or materials to inactivateone or more pathogens on the one or more surfaces or materials. Thelight includes an inactivating portion having wavelengths in a range of280 to 380 nanometers.

In one example, the one or more pathogens that are inactivated by atleast the inactivating portion of the light includes one or more ofstaphylococcus, clostridium difficile, streptococcus, or bacterialpneumonia.

In one example, the light source is configured to generate the lighttoward the one or more surfaces or materials while also concurrentlyexposing one or more human beings to the light.

In one example, the light source is configured to generate the lighttoward the one or more surfaces or materials to inactivate the one ormore pathogens without using a photosensitizer.

In one example, the light source is configured to generate the light sothat the inactivating portion of the light is imperceptible to a humanobserver of the light.

In one example, the light source is configured to generate theinactivating portion of the light such that the inactivating portion ofthe light includes no more than 0.001 watts of actinic ultraviolet lightper square meter of floor area.

In one example, the light source is configured to generate theinactivating portion of the light such that the inactivating portion ofthe light includes no more than 10 watts per square meter of floor areaof ultraviolet A light.

In one example, the light source is configured to generate theinactivating portion of the light such that the inactivating portion ofthe light includes no more than 100 watts of blue light per steradianper square meter of floor area.

In one example, the light source is configured to generate the lightsuch that a peak wavelength of the inactivating portion of the light isgreater than 300 nanometers.

In one example, the light source is configured to generate the lightsuch that the inactivating portion of the light includes wavelengths ina range of 320 to 380 nm.

In one example, the light source is configured to generate the lightsuch that the inactivating portion of the light has a peak wavelength ina range of 320 to 370 nm.

In one example, the light source is configured to generate the lightsuch that the inactivating portion of the light is pulsed at a frequencyexceeding 100 hertz with a duty factor of less than 0.5.

In one example, the light source is configured to generate the lightsuch that the inactivating portion of the light is pulsed at a frequencyexceeding 100 hertz with a duty factor of less than 0.1.

In one example, the light source generates the light to include theinactivating portion of the light with wavelengths in a range of 280 to380 nanometers, and a second portion of the light having longerwavelengths.

In one embodiment, a method for inactivating one or more pathogens andoptionally concurrently illuminating a room having one or more humanoccupants in to while the pathogens are inactivated is provided. Themethod includes generating light from a light source toward one or moresurfaces or materials to inactivate the one or more pathogens on the oneor more surfaces or materials. The light is generated with aninactivating portion of the light including wavelengths in a range of280 to 380 nanometers.

In one example, the one or more pathogens that are inactivated by atleast the inactivating portion of the light includes one or more ofstaphylococcus, clostridium difficile, streptococcus, or bacterialpneumonia.

In one example, the light source is configured to generate the lighttoward the one or more surfaces or materials while also concurrentlyexposing one or more human beings to the light.

In one embodiment, a lighting system includes a light source configuredto generate light toward one or more surfaces or materials to inactivateone or more pathogens on the one or more surfaces or materials. Thelight source is configured to generate the light at a power density ofno more than five watts per square meter at an exposed area of the oneor more surfaces or materials with an inactivating portion of the lightincluding wavelengths in a range of 280 to 380 nanometers, including nomore than 0.001 watts of actinic ultraviolet light per square meter offloor area, including no more than 10 watts per square meter of floorarea of ultraviolet A light, and including no more than 100 watts ofblue light per steradian per square meter of floor area.

In one example, the one or more pathogens that are inactivated by atleast the inactivating portion of the light includes one or more ofstaphylococcus, clostridium difficile, streptococcus, or bacterialpneumonia.

In one example, the light source is configured to generate the lighttoward the one or more surfaces or materials while also concurrentlyexposing one or more human beings to the light.

In one example, the light source is configured to generate the light sothat the inactivating portion of the light is imperceptible to a humanobserver of the light.

The foregoing description of certain embodiments of the inventivesubject matter will be better understood when read in conjunction withthe appended drawings. The various embodiments are not limited to thearrangements and instrumentality shown in the drawings. The abovedescription is illustrative and not restrictive. For example, theabove-described embodiments (and/or aspects thereof) may be used incombination with each other. In addition, many modifications may be madeto adapt a particular situation or material to the teachings of theinventive subject matter without departing from its scope. While thedimensions and types of materials described herein are intended todefine the parameters of the inventive subject matter, they are by nomeans limiting and are exemplary embodiments. Other embodiments may beapparent to one of ordinary skill in the art upon reviewing the abovedescription. The scope of the inventive subject matter should,therefore, be determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled.

In the appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the respective terms “comprising” and“wherein.” Moreover, in the following claims, the terms “first,”“second,” and “third,” etc. are used merely as labels, and are notintended to impose numerical requirements on their objects. Further, thelimitations of the following claims are not written inmeans-plus-function format and are not intended to be interpreted basedon 35 U.S.C. §112(f), unless and until such claim limitations expresslyuse the phrase “means for” followed by a statement of function void offurther structure. And, as used herein, an element or step recited inthe singular and proceeded with the word “a” or “an” should beunderstood as not excluding plural of said elements or steps, unlesssuch exclusion is explicitly stated. Furthermore, references to “oneembodiment” of the inventive subject matter are not intended to beinterpreted as excluding the existence of additional embodiments thatalso incorporate the recited features. Moreover, unless explicitlystated to the contrary, embodiments “comprising,” “including,” or“having” an element or a plurality of elements having a particularproperty may include additional such elements not having that property.

This written description uses examples to disclose several embodimentsof the inventive subject matter and also to enable a person of ordinaryskill in the art to practice the embodiments of the inventive subjectmatter, including making and using any devices or systems and performingany incorporated methods. The patentable scope of the inventive subjectmatter is defined by the claims, and may include other examples thatoccur to those of ordinary skill in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal languages of the claims.

What is claimed is:
 1. A system comprising: a light source configured togenerate light toward one or more surfaces or materials to inactivateone or more pathogens on the one or more surfaces or materials, thelight including an inactivating portion having wavelengths in a range of280 to 380 nanometers.
 2. The system of claim 1, wherein the one or morepathogens that are inactivated by at least the inactivating portion ofthe light includes one or more of staphylococcus, clostridium,streptococcus, or bacterial pneumonia.
 3. The system of claim 1, whereinthe light source is configured to generate the light toward the one ormore surfaces or materials while also concurrently exposing one or morehuman beings to the light.
 4. The system of claim 1, wherein the lightsource is configured to generate the light toward the one or moresurfaces or materials to inactivate the one or more pathogens withoutusing a photosensitizer.
 5. The system of claim 1, wherein the lightsource is configured to generate the light so that the inactivatingportion of the light is imperceptible to a human observer of the light.6. The system of claim 1, wherein the light source is configured togenerate the inactivating portion of the light such that theinactivating portion of the light includes no more than 0.00015 watts ofactinic ultraviolet light per square meter of floor area
 7. The systemof claim 1, wherein the light source is configured to generate theinactivating portion of the light such that the inactivating portion ofthe light includes no more than 0.5 watts per square meter of floor areaof ultraviolet A light.
 8. The system of claim 1, wherein the lightsource is configured to generate the inactivating portion of the lightsuch that the inactivating portion of the light includes no more than 11watts of blue light per steradian per square meter of floor area.
 9. Thesystem of claim 1, wherein the light source is configured to generatethe inactivating portion of the light such that the inactivating portionof the light includes no more than 0.001 watts of actinic ultravioletlight per square meter of floor area.
 10. The system of claim 1, whereinthe light source is configured to generate the inactivating portion ofthe light such that the inactivating portion of the light includes nomore than 10 watts per square meter of floor area of ultraviolet A lightper square meter of floor area.
 11. The system of claim 1, wherein thelight source is configured to generate the inactivating portion of thelight such that the inactivating portion of the light includes no morethan 100 watts of blue light per steradian per square meter of floorarea.
 12. The system of claim 1, wherein the light source is configuredto generate the inactivating portion of the light such that theinactivating portion of the light includes no more than 0.003 watts ofactinic ultraviolet light per square meter of floor area.
 13. The systemof claim 1, wherein the light source is configured to generate theinactivating portion of the light such that the inactivating portion ofthe light includes no more than 33 watts per square meter of floor areaof ultraviolet A light per square meter of floor area.
 14. The system ofclaim 1, wherein the light source is configured to generate theinactivating portion of the light such that the inactivating portion ofthe light includes no more than 10,000 watts of blue light per steradianper square meter of floor.
 15. The system of claim 1, wherein the lightsource is configured to generate the light such that a peak wavelengthof the inactivating portion of the light is greater than 300 nanometers.16. The system of claim 1, wherein the light source is configured togenerate the light such that the inactivating portion of the lightincludes wavelengths in a range of 320 to 380 nm.
 17. The system ofclaim 1, wherein the light source is configured to generate the lightsuch that the inactivating portion of the light has a peak wavelength ina range of 320 to 370 nm.
 18. The system of claim 1, wherein the lightsource is configured to generate the light such that the inactivatingportion of the light is pulsed at a frequency exceeding 100 hertz with aduty factor of less than 0.5.
 19. The system of claim 1, wherein thelight source is configured to generate the light such that theinactivating portion of the light is pulsed at a frequency exceeding 100hertz with a duty factor of less than 0.1.
 20. The system of claim 1,wherein the light source generates the light to include the inactivatingportion of the light with wavelengths in a range of 280 to 380nanometers, and a second portion of the light having longer wavelengths.21. A method comprising: generating light from a light source toward oneor more surfaces or materials to inactivate one or more pathogens on theone or more surfaces or materials, the light generated at a powerdensity of no more than five watts per square meter at an exposed areaof the one or more surfaces or materials with an inactivating portion ofthe light including wavelengths in a range of 280 to 380 nanometers. 22.The method of claim 21, wherein the one or more pathogens that areinactivated by at least the inactivating portion of the light includesone or more of staphylococcus, clostridium difficile, streptococcus, orbacterial pneumonia.
 23. The method of claim 21, wherein the lightsource is configured to generate the light toward the one or moresurfaces or materials while also concurrently exposing one or more humanbeings to the light.
 24. A system comprising: a light source configuredto generate light toward one or more surfaces or materials to inactivateone or more pathogens on the one or more surfaces or materials, thelight source configured to generate an inactivating portion of the lightincluding wavelengths in a range of 280 to 380 nanometers, including nomore than 0.001 watts of actinic ultraviolet light per square meter offloor area, including no more than 10 watts per square meter of floorarea of ultraviolet A light, and including no more than 100 watts ofblue light per steradian per square meter of floor area.
 25. The systemof claim 24, wherein the one or more pathogens that are inactivated byat least the inactivating portion of the light includes one or more ofstaphylococcus, clostridium difficile, streptococcus, or bacterialpneumonia.
 26. The system of claim 24, wherein the light source isconfigured to generate the light toward the one or more surfaces ormaterials while also concurrently exposing one or more human beings tothe light.
 27. The system of claim 24, wherein the light source isconfigured to generate the light so that the inactivating portion of thelight is imperceptible to a human observer of the light.